fix make-parameter for SRFI-39

svn: r13684

collects/srfi/39.ss

@@ -1,3 +1,18 @@
-;; Supported by core PLT:
+;; Supported by core PLT, with a slight difference in how a guard is used:
 #lang scheme/base
-(provide make-parameter parameterize)
+
+(define make-parameter*
+  (let ([make-parameter
+         (case-lambda
+          [(v) (make-parameter v)]
+          [(v guard) (make-parameter (if (and (procedure? guard)
+                                              (procedure-arity-includes? guard 1))
+                                         ;; apply guard to initial value:
+                                         (guard v)
+                                         ;; let `make-parameter' complain:
+                                         v)
+                                     guard)])])
+    make-parameter))
+
+(provide (rename-out [make-parameter* make-parameter])
+         parameterize)